The present application claims priority to Korean patent application number 2005-000100741, filed on Oct. 25, 2005, and claims priority to U.S. patent application Ser. No. 11/584,554, filed Oct. 23, 2006, both of which are incorporated by reference in their entirety.
The present invention relates to a complementary metal-oxide semiconductor (CMOS) image sensor, and more particularly, to a pixel region of a CMOS image sensor. Generally, an image sensor is a semiconductor device that converts an optical image into an electrical signal. Representative image sensors include a charge coupled device (CCD) and a complementary metal-oxide semiconductor (CMOS) image sensor.
In the CCD, respective CMOS capacitors are arranged such that they are very close to one another, and charge carriers are stored at the capacitors and they are transferred. The CMOS image sensor utilizes a CMOS technology using a control circuit and a signal processing circuit as a peripheral circuit, and a plurality of MOS transistors corresponding to each number of unit pixels are formed for sequential outputs.
FIG. 1 illustrates a circuit diagram of a typical CMOS image sensor, and more particularly, a circuit diagram of a pixel.
The pixel of the typical CMOS image sensor includes: a photodiode PD, a transfer transistor Tx, a reset transistor Rx, a drive transistor Dx, and a select transistor Sx. The photodiode PD generates photocharges corresponding to incident light and transfers the photocharges. The transfer transistor Tx transfers the photocharges supplied from the photodiode PD to a floating diffusion node FD. The reset transistor Rx resets the floating diffusion node FD, and the drive transistor Dx drives a source terminal in response to a voltage supplied to the floating diffusion node FD. The select transistor Sx is connected to the source terminal of the drive transistor Dx and selectively connects the source terminal to an output terminal using the drive transistor Dx.
In particular, a region ‘A’ including the floating diffusion node FD, where the transfer transistor Tx and the drive transistor Dx are commonly connected, performs a very important role in the CMOS image sensor operation.
FIG. 2 illustrates a layout diagram of the circuit diagram illustrated in FIG. 1. Gate patterns of transistors configuring a pixel are denoted with the same denotations as the corresponding transistors for convenience.
A photodiode PD and an active region are formed. A gate pattern Tx of a transfer transistor, a agate pattern Rx of a reset transistor, a gate pattern Dx of a drive transistor, and a gate pattern Sx of a select transistor are sequentially disposed.
Contacts CT2, CT3, and CT6 contacting each of the gate patterns Dx, Sx, and Tx respectively, and contacts CT1, CT4, and CT5 contacting the active region are disposed as illustrated.
The contact CT1, contacting a floating diffusion node in the active region, and the contact CT2, contacting the gate pattern Dx, are connected to each other through a metal line M1A. A metal line M1B is connected to the contact CT6 contacting the gate pattern Tx.
FIG. 3A illustrates a cross-sectional view of a typical CMOS image sensor and FIG. 3B illustrates a top view of the typical CMOS image sensor. FIGS. 3A and 3B show typical limitations of the typical CMOS image sensor.
Referring to FIG. 3A, device isolation structures STI are formed. Then, a photodiode including an N-type region DN and a P-type region PO, a gate pattern Tx of a transfer transistor, a gate pattern Rx of a reset transistor, a gate pattern Dx of a drive transistor, and a gate pattern Sx of a select transistor are sequentially formed.
Referring to FIG. 3B, the gate patter Tx is disposed adjacent to the photodiode. The gate pattern Rx and the gate pattern Dx are disposed adjacent to the gate pattern Tx.
A contact is generally required to be formed at each region of a floating diffusion node FD and the gate pattern Dx in order to connect the floating diffusion node FD and the gate pattern Dx to each other. A metal line is generally required to connect the contacts. A misalignment defect may occur at a region B when forming the contact due to the large scale of integration of the device.
A typical floating diffusion node in a CMOS image sensor generally functions to read a signal provided by an electron that is generated by light irradiated upon a pixel. A voltage supplied to the floating diffusion node determines the driving capability of a drive transistor.
Thus, a line between the floating diffusion node and a gate pattern of the drive transistor is very important in transferring a precision signal.
Two contacts including aluminum are sometimes formed at a floating diffusion node region in a CMOS image sensor using the 0.18 —— technology to transfer a more precise signal.
However, it is hard to dispose two contacts at the floating diffusion node if the CMOS image sensor is applied in a portable product due to the decreased size and increased number of pixels.
It is difficult to dispose more than one contact at a floating diffusion node of a pixel while decreasing the size of a CMOS image sensor and increasing the number of pixels. Furthermore, it is becoming hard for even one single contact to form stably because a circuit area of the floating diffusion node is decreasing.
This result is derived because an overlay margin is decreased during the contact formation performed at the floating diffusion node.
A stable connection between the floating diffusion node and the gate pattern of the drive transistor is a core function of image sensor operations. Instability in signal transfer can be removed by a stable connection.